The R-T-B based permanent magnet (R is rare earth element(s), T is Fe or Fe with part of which is replaced with Co, and B is boron) having the tetragonal compound R2T14B as the main phase is known to have excellent magnetic properties and has been a representative permanent magnet with high performance since the invention in 1982 (Patent document 1: JPS59-46008A).
The R-T-B based magnets with the rare earth element(s) R being consisted of Nd, Pr, Dy, Ho and Tb have a large anisotropy magnetic field Ha and are preferably used as permanent magnet materials. Among them, the Nd—Fe—B based magnet having Nd as the rare earth element R is widely used because it has a good balance among saturation magnetization Is, Curie temperature Tc and anisotropy magnetic field Ha, and is superior in resource abundance and corrosion resistance than R-T-B based magnets using other rare earth elements as R.
As a rotating machine widely used in consumer, industry, transportation equipment, permanent-magnet synchronous rotating machines tend to be used extensively in terms of saving energy and energy density in recent years.
For a permanent-magnet synchronous motor, it is provided with permanent magnets in the rotor. If permanent magnets with rare earth element Nd (which has a large specific gravity) occupying about ⅓ of their masses like Nd—Fe—B based magnets are used, a problem will exist that the moment of inertia will increase because of the increasing of rotor weight. That means it will cause a reduction in controllability and efficiency.